The present invention relates to a photoresist composition showing high sensitivity and having improved adhesion to a substrate.
Photoresist compositions are widely used for plate making and photo-etching and must have the following characteristics: (a) high resolution, (b) high sensitivity, (c) stability quality in preservation, (d) a sufficiently hard photoresist layer to resist corrosion and abrasion, (e) adhesion to a substrate is strong and having a wide latitude in developing, (f) both the photoresist itself and developing solution should not cause pollution.
Conventionally a dichromate series photoresist made of a mixture of dichromate and a water-soluble polymer, for example glue, gelatin, albumin (egg white) and polyvinyl alcohol, is used widely for plate making and photo etching. Dichromate, Cr.sub.2 O.sub.7.sup.2- is used to harden the polymer. The reason is that this photoresist is cheap and approximately meets the above-mentioned criteria (a), (b), (d) and (e). However, this type of photoresist gradually changes its quality by a natural reaction, the so-called "dark reaction", even though it is stored in the dark, and the photoresist becomes unusable. Further, there are pollution problems if chromium is used as the cross-linking agent.
To resolve the above-mentioned drawbacks, it has been suggested to make a photoresist composition by introducing a photosensitivity radical into the polymeric material itself or by mixing an organic photosensitive agent with a polymeric material.
As one of these attempts, a photoresist comprising a water-soluble polymer mixed with a light-sensitive organic cross-linking agent instead of dichromate is known. See Japanese Patent Publication No. 48,946/1976. For example, a photoresist composition comprised a water-soluble bisazide compound having a hydrophilic radical, for example 4,4'-bisazidostilbene-2,2'-disulfonic acid or its salt, and a water-soluble polymer, for example polyacrylamide, polyvinylpyrrolidone or gelatin, is known. This bisazide photoresist varies its characteristics according to the characteristics of a water-soluble polymer combined. This photoresist approximately meets the above-mentioned criteria (a), (b), (c) and (e). However, this type of photoresist generally has weak adhesion to the substrate. Weak adhesion causes low resolution. That is, to obtain an adequate resolution, strict developing control is required. Overdeveloping causes a hardened photoresist layer to peel off from the substrate while, in contrast, a weak developing causes the unhardened part to remain on the substrate which is also undesirable. The bisazide-type photoresists also cannot be formed into a pattern if it is exposed to light below a predetermined value. In other words these photoresists have the characteristics of reciprocity law failure, so that as a consequence the bisazide-type photoresists are not widely used for practical reasons.
Generally, a bisazide photoresist has weaker adhesion to the substrate than a dichromate photoresist. The reason is unclear, however, it can be considered that in a dichromate photoresist a chrome ion acts as a connector for connecting a polymer with the substrate. On the contrary, a bisazide photoresist has no such function.
Therefore, in order to remove this drawback a method for improving the adhesion has been proposed. Japanese Patent Publication No. 19,982/1976 discloses the photoresist composition comprising a bisazide compound, a suitable water-soluble polymer and at least one silane compound selected from the .gamma.-aminopropyltriethoxysilane, N-(.beta.-aminoethyl)-.gamma.-aminopropyltrimethoxysilane and N-(.beta.-aminoethyl)-.gamma.-aminopropyltrimethoxysilane as an adhesive activator. However, a bisazide photoresist containing the silane compounds still does not provide adequate adhesion.